A pedicle screw is a surgical instrument/implant for stabilizing the vertebral column. In backbone surgery, pedicle screws are frequently used for stabilizing operations such as for repositioning a sliding vertebra or for stiffening vertebrae. Their usually self-tapping thread facilitates the process of screwing the screw in the vertebral arch roots of two or more vertebrae. The screw head or the tulip is either supported on the screw shaft in movable fashion (polyaxial) by means of a ball joint or rigidly connected (mono-axial) to the thread.
Provided in the screw head/tulip is an axially extending, U-shaped recess capable of receiving a rod. Said rod is fixed in the screw head/tulip by means of a set screw. Hence, the pedicle screw may be used as an instrument to return a displaced vertebra (sliding vertebra) back to the correct position, for example. For stabilizing the vertebral column, four or more pedicle screws are interconnected by means of rods along the backbone axis. Pedicle screws having the previously described construction are known in the prior art from many disclosures to which reference is made in the following description. This is why a repeated description of the pedicle screw, in particular of the tulip and the traverse, may be omitted with reference to said commonly known prior art.
Regarding the screw shaft, however, pedicle screw shapes are known from prior art which have different core cross-sections (expanding continuously or in step-wise manner) in the longitudinal direction of the shaft. This measure is supposed to ensure that a pedicle screw which is screwed in the vertebral arch root gets radially braced in particular in the outer/proximal head zone in the borehole and thus is able to transmit higher forces into the vertebra without coming loose. It was also contemplated to radially expand the screw flanks in the longitudinal direction of the shaft toward the screw head in a continuous or step-wise manner in order to achieve an increasing incising effect in the vertebral body. This is done with the aim to prevent the pedicle screw from breaking out.
Despite of these commonly known efforts in terms of providing an optimum design of the shaft and the screw flanks, the urgent problem of the reliable grip of the pedicle screw in the vertebra over a long period of time continues to exist. In particular, following defects in the prior art turn out to be particularly fatal:                An insufficient secondary stability with respect to rotational forces acting on the pedicle screw,        if the pedicle screw is torn out of the bone in case of tensile and/or shearing forces,        stiff and slow screwing process,        uncontrollability of the screwing process.        
In fact, there is a variety of different screw shapes and screw constructions in the field of screw technology, but these are designed outside the field of medical engineering with regard to specific applications such as cooperating with wood, plastics, plasterboards and the like construction materials. Bone material is clearly different from these, not only due to its composition and strength but also because of the fact that it is a material which is still alive and constantly changes and renews its structure. In addition, conventional screws outside the field of medical engineering are exposed to other loads (mostly static loads) than in a medical application in the body of a patient. This is why they can be made from other materials which result in an optimum connection with the construction material concerned. In medical technology, this is not possible (or only partly possible) due to reasons of hygiene as well as biocompatibility. This is why technical solutions outside the field of medical engineering can not be readily transferred to bone screw constructions.